Techniques currently used for cancer diagnosis mainly include (i) those using large-sized testing instruments (e.g., MRI) and (ii) those for measuring tumor markers or the like in blood, and expectations are now focused on (ii) which are simple techniques with less burden on patients. In particular, cancer cells circulating in the peripheral blood of cancer patients (i.e., circulating tumor cells (CTCs)) show a close relationship with clinical symptoms because these cells increase the risk of systemic metastasis and because the prognosis of patients with CTCs is significantly poor. Thus, it has been expected to develop a technique for simple and highly sensitive detection of CTCs as a predictive factor or surrogate marker for prognosis.
Techniques used for CTC detection include detection with a cancer-related antigen such as EpCAM (epithelial cell adhesion molecule) or cytokeratin-8 (e.g., CellSearch system) and detection by means of RT-PCR, etc. However, these cancer-related antigens are also expressed on normal epithelial cells and hence are highly likely to cause false positive detection, while cell morphology characteristic of cancer cells cannot be observed at the same time in the case of PCR detection. For these reasons. there has been a demand for a new technique in terms of sensitivity, simplicity, accuracy and costs.
On the other hand, the inventors of the present invention have already developed a conditionally replicating adenovirus which grows specifically in cancer cells and expresses GFP (GFP-expressing conditionally replicating adenovirus: GFP-CRAd) (which is referred to as TelomeScan®, OBP-401 or Telomelysin-GFP) (Patent Document 1: WO2006/036004). Moreover, the inventors of the present invention have also developed a simple technique for CTC detection using this TelomeScan (Non-patent Document 1: Kojima T, et al, J. Clin. Invest., 119: 3172, 2009).
However, since TelomeScan has the fiber protein of adenovirus type 5 and infects via coxsackievirus and adenovirus receptor (CAR) in target cells, TelomeScan may not infect cells which do not express CAR. In particular, it is known that CAR expression is reduced in highly malignant cancer cells which are highly invasive. metastatic and proliferative (Non-patent Document 2: Okegawa T., et al, Cancer Res., 61: 6592-6600, 2001); and hence TelomeScan may not detect these highly malignant cancer cells. Moreover, although less likely. TelomeScan may give false positive results by infecting and growing in normal blood cells (e.g., leukocytes) to cause GFP expression.
For these reasons, there has been a demand for a reagent for cancer cell detection and a reagent for cancer diagnosis, each of which detects almost all cancer cells including CAR-negative ones and does not give any false positive results in normal blood cells.